Engines with rotating components are subject to vibratory forces at frequencies which are related to the angular velocity of the respective component and hence engine speed. These frequencies are conventionally known as engine order forcing frequencies, each engine order corresponding to a rotational frequency of a particular component (or harmonic thereof) and exerting a corresponding vibratory force on the engine. Where there is gearing between components, non-integral engine order frequencies can arise, i.e. frequencies which are related to the primary component rotational frequency, but not necessarily by an integer number.
The forces may arise because e.g. an engine is out of balance on a particular shaft, stiffness irregularities in engine components, shaft mis-alignment, and (significantly in the case of gas turbine engines) aerodynamic interactions between the blades of the engine.
At a given engine speed, a number of these engine orders are generally active and result in corresponding vibration responses in the engine which are measurable e.g. as strains, velocities or accelerations. Each vibration response generally has approximately the same frequency as the engine order forcing frequency which generated it. For steady state conditions (e.g. constant speed), the frequencies will be the same. However, the relative phase between a vibration response and the corresponding engine order may change as the engine speed varies, and particularly when the engine order traverses a resonance frequency of the engine. Indeed, merely moving toward or away from such a resonance may cause the relative phase to change. In particular, under reasonably slow engine speed changes, engine order forcing frequencies and their corresponding vibration response frequencies tend to be very close, but usually varying sufficiently to allow some relative changes near resonances. On the other hand, the ratios between the engine order forcing frequencies generally remain constant as the engine speed varies.
Quantification of active engine order forcing frequencies and phase changes can be helpful for engine operators attempting to identify modes of vibration. These in turn may be useful for understanding engine behaviour, providing validation for engine models, and engine troubleshooting and “health” monitoring.
A conventional approach for determining engine order forcing frequencies is to measure engine component (e.g. shaft) rotational speeds directly using tachometer-like measuring devices. This approach is relatively simple in concept, but relies on being able to provide accurate and robust measuring devices and to appropriately position the devices in the engine. This may cause difficulties in practice.
The present invention is based, at least partly, on the realisation that known relations between engine order forcing frequencies and vibration response frequencies allow other engine order forcing frequencies, and optionally engine order/vibration response relative phase changes, to be determined indirectly from measurements of vibration responses.